Multipurpose Host System for Invasive Cardiovascular Diagnostic Measurement Acquisition and Display

ABSTRACT

An ultrasound catheter is described herein for insertion into a cavity such as a blood vessel to facilitate imaging within a vasculature. The catheter comprises an elongate flexible shaft, a capacitive microfabricated ultrasonic transducer, and a sonic reflector. The elongate flexible shaft has a proximate end and a distal end. A capacitive microfabricated ultrasonic transducer (cMUT) is mounted to the shaft near the distal end. The reflector is positioned such that a reflective surface redirects ultrasonic waves to and from the transducer. In other embodiments, the catheter comprises a plurality of cMUT elements and operates without the use of reflectors. In further embodiments, integrated circuitry is incorporated into the design.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Hossack et al. U.S. provisionalapplication Ser. No. 60/677,876 filed on May 5, 2005, entitled “cMUTIVUS CATHETERS,” the contents of which are expressly incorporated hereinby reference in their entirety including the contents and teachings ofany references contained therein.

BACKGROUND OF THE INVENTION

Intravascular Ultrasound (IVUS) has become an important interventionaldiagnostic procedure for imaging atherosclerosis and other vesseldiseases and defects. In the procedure, an IVUS catheter is threadedover a guidewire into a blood vessel of interest, and images areacquired of the atherosclerotic plaque and surrounding area usingultrasonic echoes. This information is much more descriptive than thetraditional standard of angiography, which only shows an image of theblood flowing through the vessel lumen. Some of the key applications ofIVUS include: determining a correct diameter and length of a stent tochoose for dilating an arterial stenosis, verifying that a post-stentingdiameter and luminal cross-section area are adequate, verifying that astent is well apposed against a vessel wall to minimize thrombosis andoptimize drug delivery (in the case of a drug eluting stent) andidentifying an exact location of side-branch vessels. In addition, newtechniques such as virtual histology (RF signal-based tissuecharacterization) show promise of aiding identification of vulnerableplaque (i.e., plaque which is prone to rupture and lead to onset of aheart attack).

There are generally two standard types of IVUS catheters, rotationalIVUS catheters and phased array catheters. In a rotational IVUScatheter, a single transducer consisting of a piezoelectric crystal isrotated at approximately 1800 revolutions per minute while the elementis excited by a signal. This causes the element to vibrate at afrequency from around 9 to 45 MHz, and above depending on the dimensionsand characteristics of the transducer. The single element transducer ofthe rotational IVUS catheter can be made very thin and therefore able tovibrate at relatively high frequencies, thus achieving a relatively highresolution, especially in the near field (close to the outside diameterof the catheter sheath). In addition, this type of transducer can beexcited by a relatively high Voltage, increasing the signal to noiseratio. Because the transducer rotationally sweeps past the guidewireduring each rotation, a guidewire shadow is seen in the image thatobscures some of the image of tissue in back of the guidewire. Inaddition, rotation of the transducer, usually achieved by a reinforcedcoil drive shaft, can be uneven and cause distortion of the image. Thiseffect is known as NURD (non-uniform rotational distortion).

Another type of IVUS catheter is a phased array (or solid state)catheter. This catheter has no rotating parts, but instead has amulti-element transducer (for example 64 elements), in which eachelement is fired in a specific order by means of several smallintegrated circuits in the tip of the catheter. The multiplexing anddemultiplexing performed by the integrated circuits allows for a minimalnumber of wires inside the catheter. Due to its structure, this type ofcatheter requires little or no prepping (e.g. flushing with saline toremove air bubbles from within the catheter) and is very flexible andtrackable over a guidewire. It is a difficult, multi-step process tomake this multi element transducer. One of the challenges of this methodis that it is difficult to make the elements thin enough to achievefrequencies as high as those utilized in rotational IVUS catheters.

In the last decade, a new technology has shown promise in ultrasoundtransducers. This technology is known as cMUT (capacitiveMicrofabricated Ultrasonic Transducer). The cMUT transducers typicallyconsist of an array of tiny drums fabricated on silicon or othersemi-conductor materials. In the cMUT manufacturing process, a thinsacrificial layer is first deposited in a desired pattern. A thinnitride layer is then deposited over the sacrificial layer. This willform both the “drum shell” (bottom and cylinder) and the “drum head”(membrane). Tiny holes are etched through the nitride layer, allowingthe sacrificial layer to be removed. The nitride layer is then sealedand an electrical connection is made, so that the membrane can beexcited, causing it to vibrate. Typically, a bias DC Voltage is appliedto keep the drum from collapsing. New techniques, however, apply the DCbias Voltage to maintain the membrane in a controlled, imploded (orcollapsed) state. An AC Voltage is also applied to create the ultrasoundenergy by inducing vibration within the drum head (membrane). Inaddition, signal processing circuitry can be included in the siliconbase of the cMUT structure. The cMUT transducer shows promise for lowercost fabrication because of the consistency of semiconductor processingtechnology.

BRIEF SUMMARY OF THE INVENTION

An ultrasound catheter is disclosed herein for insertion into a bodycavity, such as a blood vessel. The catheter comprises an elongateflexible shaft, a capacitive microfabricated ultrasonic transducer, anda sonic reflector. The elongate flexible shaft has a proximal end and adistal end. The capacitive microfabricated ultrasonic transducer ismounted to the shaft near the distal end. The reflector is positionedsuch that a reflective surface redirects ultrasonic waves to and fromthe transducer. Particular embodiments do not include the reflector, andinstead transmit and receive ultrasound signals directly into an imagedmedium.

In a disclosed embodiment, the ultrasound catheter comprises an elongateflexible shaft and a probe, mounted upon the flexible shaft, comprisinga plurality of capacitive microfabricated ultrasonic transducers. Theelongate flexible shaft has a proximal end and a distal end. Theplurality of capacitive microfabricated ultrasonic transducers iscoupled to the shaft near the distal end.

In yet another disclosed embodiment, the ultrasound catheter comprisesan elongate flexible shaft, a capacitive microfabricated ultrasonictransducer, and an integrated circuit. The elongate flexible shaft has aproximal end and a distal end. The capacitive microfabricated ultrasonictransducer module is mounted to the shaft near the distal end. Theintegrated circuit, interposed between a connector at the proximal endand the cMUT devices at the distal end, performs at least a multiplexingfunction with regard to signal lines from elements of the transducermodule.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a cMUT IVUS catheter;

FIG. 2 is a fragmentary perspective view of the cMUT IVUS catheter ofFIG. 1;

FIG. 3 is another fragmentary perspective view of the cMUT IVUS catheterof FIG. 1;

FIG. 4 is a fragmentary partial cross-sectional side view of a cMUT IVUScatheter disposed within a blood vessel;

FIG. 5 is a fragmentary perspective view of an embodiment of areflective cMUT IVUS catheter as viewed from the distal end;

FIG. 6A is a fragmentary side view of the reflective cMUT IVUS catheterof FIG. 5;

FIG. 6B is an enlarged fragmentary view of the reflective cMUT IVUScatheter of FIG. 5, showing representative reflections of ultrasoundwaves by a sonic mirror;

FIG. 6C is another enlarged fragmentary view of the reflective cMUT IVUScatheter of FIG. 5, showing the catheter in a flexed configuration;

FIG. 6D is another enlarged fragmentary view of another embodiment of areflective cMUT IVUS catheter wherein the sonic mirror is separable intotwo elements;

FIG. 7 is a fragmentary perspective view of an embodiment of an innertube of a cMUT IVUS catheter comprising conductive ridges;

FIG. 8 is a fragmentary perspective view if another embodiment of aninner tube of a cMUT NUS catheter having a non-ridged section;

FIG. 9 is a perspective view of a modular cMUT ring;

FIG. 9A is a perspective view of another embodiment of a modular cMUTring with an alternative conductive path arrangement;

FIG. 10 is another perspective view of the modular cMUT ring of FIG. 9;

FIG. 11 is a side view of the modular cMUT ring of FIG. 9;

FIG. 12 is a fragmentary perspective view of another embodiment of areflective cMUT IVUS catheter;

FIG. 13 is a side view of another embodiment of a modular cMUT ring;

FIG. 14 is a perspective view of another embodiment of a modular cMUTring with a plurality of drums;

FIG. 15 is a fragmentary side view of another embodiment of a reflectivecMUT IVUS catheter suitable for side and forward viewing;

FIG. 16 is a side view of another embodiment of a modular cMUT ring;

FIG. 17 is a side view of the reflective cMUT catheter of FIG. 15showing the shape of the sonic field during the firing of a selectivenumber of cMUT rings;

FIG. 18 is another side view of the reflective cMUT catheter of FIG. 15showing the firing of the foremost cMUT ring only;

FIG. 19 is another embodiment of a reflective cMUT IVUS catheterparticularly suitable for forward looking procedures;

FIG. 20 is a fragmentary perspective view of an embodiment of a directcMUT IVUS catheter;

FIG. 21 is another embodiment of a direct cMUT IVUS catheter;

FIG. 22 is another embodiment of a direct cMUT IVUS catheter, thecatheter having staggered drum orientation angles;

FIG. 23 is a cross-sectional view of an embodiment of a cMUT ring shownwith the direct cMUT IVUS of FIG. 20; and

FIG. 24 is another embodiment of a cMUT ring for a direct cMUT IVUScatheter.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to cMUT IVUS devices that are mounted to distalends of catheters. The cMUT IVUS devices are presented in two differentgeneral configurations, reflective and non-reflective. Reflective cMUTIVUS devices contain one or more cMUT rings and one or more sonicmirrors. The ultrasound is generated by the cMUT rings and reflected bythe sonic mirrors in desired directions for imaging blood vessels andtheir disease. Non-reflective cMUT IVUS probes do not have the sonicmirror elements, but rather transmit the ultrasound directly from thecMUT membranes in the desired direction.

FIG. 1 shows a cMUT IVUS catheter 10 having a catheter shaft 20, amanifold 30, a cable 40 and an electrical connector 50. The cathetershaft 20 includes a distal end 60 and a proximal end 70. The proximalend 70 includes a strain relief 80. The usable length of the cathetershaft (the portion that can be inserted into a patient) can be varieddepending upon the application. For example, in a coronary arteryapplication, a length of 135 cm to 150 cm is desirable. The manifold 30is an area for transition between the portion of the catheter that isinserted and the portion that remains outside the body. The manifold canalso include one or more luer connectors (not shown in this embodiment)to facilitate flushing of the catheter, usually with sterile saline. Thedistal end 60 of the catheter shaft is the portion which, in theillustrative embodiments, includes the transducers. It is this sectionthat is tracked to the target area in the blood vessel for imaging.Though in some applications, it is not necessary to track the catheterover a guidewire, in most cases, a guidewire lumen is needed in thecatheter to allow guidewire tracking of this nature. In the coronaryartery application, a preferred guidewire lumen configuration isillustrated in FIG. 1. Though the guidewire is not shown, the guidewirelumen extends from the distal guidewire port 90 to the proximalguidewire port 100. In some embodiments, keeping this length betweenabout 20 cm and 30 cm facilitates a single operator easilyadvancing/withdrawing the catheter over the guidewire. The proximalguidewire port 100 remains protected inside a separate guiding catheterduring the procedure.

In other embodiments of a guidewire lumen, the proximal guidewire port100 is located at the proximal end of the manifold 30. Thisconfiguration allows for exchanges of guidewires during the procedure aswell as the ability to flush the lumen. This type of catheter (known as“over-the-wire”) may not be as desirable for single operator use as theembodiment pictured in FIG. 1, however, because of the long length ofguidewire and catheter engagement that needs to be manipulated outsideof the body. It will be appreciated that the catheter and the guidewiremay have any suitable length.

Referring again to FIG. 1, the electrical connector 50 can be attachedto an IVUS console or to a patient interface module, allowing forbi-directional signal transfer. Electrical wires or a flex circuit arecontained in the cable 40 to connect the electrical connector 50 withthe rest of the catheter 10. Detail of the electrical connector 50 canbe seen in FIG. 3. The electrical connector 50 consists of a housing 180and electrical connections 160, which can consist of a series of one ormore pins 170. The connector 50 snaps onto the IVUS console or patientinterface module in a frictional and/or locking fashion. The console orpatient interface module can be configured to allow more than one ofthese connectors to attach (e.g., from two different types ofcatheters).

FIG. 2 shows details of the distal portion of the catheter 10 depictedin FIG. 1. A modular series of capacitive microfabricated ultrasonictransducers (cMUT) 110 having one or more individual modules 120 isdisposed at the distal end 60 of the catheter shaft 20. The modules 120can be configured for side-viewing IVUS, forward-viewing IVUS or acombination of the two in the same catheter. The modular nature of thecMUT series 110 allows for this adaptability and customization andpermits greater flexibility and trackability of the catheter due to flexpoints, e.g., flex point 150 between the individual modules of the cMUTseries 110. The diameter of the cross-section of the modules making upthe cMUT series 110 is, by way of example, close in size to the diameterof the cross-section of the distal end 60 of the catheter shaft 20. Thedimension matching allows for smoother catheter tracking through tightvessel segments. A small diameter distal tip 130 allows the catheter tosmoothly enter a severely reduced diameter portion of the diseasedvessel over the guidewire, or to track through severe tortuosity. Atransition section 140 assures that the step up in diameter between thedistal tip 130 and the rest of the distal end 60 is gradual/tapered andnot abrupt. The transition section 140 therefore reduces the chancesthat the catheter will be caught/blocked when navigating through anychallenging portions of anatomy, stents or other intravascular devices.This is important in several types of anatomy in which these catheterscan be used, including, but not limited to, coronary, carotid, neuro,peripheral or venous.

FIG. 4 illustrates a cMUT IVUS catheter 10 in an operable position overa guidewire 190 in an artery 200. The atherosclerotic plaque 210 hasboth stenotic disease 220 and occlusive disease 230 in this case. Thecatheter 10 is shown emitting side-firing ultrasound 240 to image thestenotic disease 220. Acoustic echoes 250 are received which giveinformation about the nature and dimension of the stenotic disease andunderlying vessel. Similarly, the occlusive disease 230 is imaged viathe forward-firing ultrasound 260 and corresponding acoustic echoes 270.

In FIG. 5 and FIG. 6A, two views of the distal end of a first embodimentof a reflective cMUT IVUS catheter are shown. The catheter comprises oneor more cMUT rings, such as cMUT ring 280, having a forward face 300 anda backward face 310 (see, FIG. 6A). Thin-walled acoustic membranes, suchas membrane 290, are arrayed on each of the two faces of the cMUT ring280. By way of example, the membranes are substantially circular andhave a diameter of about 0.006″, but this diameter can be greater orless, depending upon the configuration. Beneath each membrane is ahollow cavity. Interspersed between the cMUT rings are sonicmirrors/reflectors, such as mirror/reflector 320, which serve to reflectthe ultrasound and redirect it. The mirrors, by way of example, have anangled front face 330 and an angled back face 340. In the configurationshown in FIGS. 5 and 6 a, the faces are angled at 45°. It will beappreciated that the faces may be disposed at any of a variety ofsuitable angles. The ultrasonic waves emitted in a forward direction bythe membranes (e.g., membrane 290) of the forward face 300 are thereforereflected to the side. Similarly, the ultrasonic waves emitted in abackward direction by the membranes of the backward face 310 arereflected to the side. This facilitates a side-viewing IVUS catheterwith several modules, such as module 120, made up of a cMUT ring 280 anda sonic mirror 320 combination. While stationary, a catheter of thisconstruction can image over a longer length than the traditional IVUScatheter, which normally would have to be pulled back during the imagingprocedure in order to image more than one axial location in a bloodvessel.

In the catheter illustrated in FIGS. 5 and 6A, the length of the modularseries 110 is approximately six millimeters, however, it will beappreciated that the modular series 110 may be any suitable length andmay comprise any suitable number of cMUT rings and mirrors. Over thissection there are nine cMUT rings and ten sonic mirrors. Only one of thetwo faces are used on the distal-most and proximal-most mirrors, theother face of each of those mirrors fitting into a groove inside thetransition section and inside the catheter shaft. Therefore, the totalnumber of mirror faces used matches that of the number of cMUT ringsides used: eighteen in this example. This means that over a sixmillimeter length, there are eighteen axially separated image planes.Each of those image planes is generated by twelve membranes (e.g.,membrane 290) of each cMUT ring face. This means that the six millimeterlength has a cylindrical array of 18 by 12 elements, or 216 totalelements. The “elemental dimensions” of such arrays will differ inaccordance with various embodiments.

In existing phased array catheter firing schemes, a single element firesa wavelet. The ensuing/corresponding echo of the wavelet is then sensedby the same element that fired the original wavelet. It is also sensedby several of the neighboring elements. This firing scheme continues forevery element, creating what is commonly referred to in the art as a“synthetic aperture.” The same sort of firing and receiving strategy isused in the modular series 110 depicted here, both in relation to thecircumferential orientation, but also in other orthogonal planes. Thewidth of the cMUT rings and the width of the sonic mirror can beconfigured so that the axial scan lines have equal spacing.

FIG. 6B depicts the path of an emitted wavelet 350 and a reflectedwavelet 360 from the membrane 290 of the cMUT ring 280. The subsequentechoes can be sensed by the same membrane 290, or by other membranes.Examples of the possible membranes are: on the other side of the samering 280 (echo 390, reflected echo 400), on a different ring 285 (echo370, reflected echo 380), or in a different circumferential orientation(echo 410, reflected echo 420), or any combination of suchside/ring/position alternatives. The circuit control scheme for a cMUTprobe device can also be configured so that certain drums only send andother drums only receive. The circuit control scheme can also beconfigured so that multiple drums fire at the same time. Multiplexingand parallel activation circuitry is, by way of example, configured inthe cMUT rings to facilitate each ring to fire and sense echoes atspecified times. Likewise, multiplexing and parallel activationcircuitry is, by way of example, configured so that selected membranesaround the circumference of the ring fire at specified times.

The modular cMUT catheter is, for example, flexible in the imagingregion because of the flex points 150 between the modules 120. Turningto FIG. 6C, it is appreciated that as the catheter flexes in the modularsection, the flex points 150 allow for a slight separation of the ringsand mirrors, thus providing a more flexible and trackable catheter. Inother embodiments, the flex points are also in the middle of the module(between a ring/mirror combination). The catheter is shown flexing dueto a curve in a blood vessel. The rings and mirror/reflectors at theflex points 150 at the top of FIG. 6C have become more separated, whilesuch separation is not present at the bottom. The incident ultrasound350 a and reflected ultrasound 360 a at the top of the figure behavedifferently from the incident ultrasound 350 b and reflected ultrasound360 b at the bottom of the figure. At the bottom of FIG. 6 c, anincidence angle 362 of the ultrasonic wave to the sonic mirror faceincreases, in turn causing the reflectance angle 368 to increaseproportionally (in comparison to a catheter in a straight configurationas described above). Also shown is the total angle between incidence andreflectance 365. At the top of FIG. 6C, an incidence angle 352 of anultrasonic wave to a sonic mirror face decreases, in turn causing areflectance angle 358 to decrease proportionally. Also shown is a totalangle between incidence and reflectance 355. In the figure, angle 355 isgreater than angle 365. In a tortuous artery, this effect serves tobetter maintain the resultant firing of the ultrasonic wave close toperpendicular to the axis of the blood vessel. Another embodiment ispresented in FIG. 611 The sonic mirror is divided into two sections thatsurround the cMUT ring 280. Proximal sonic mirror 321 and distal sonicmirror 322 provide the reflection of ultrasound waves emanating from themembranes, e.g., membrane 290 of the cMUT ring 280, however they remainstatic in relation to the cMUT ring so that the incidence andreflectance angles remain constant. The module 120 remains together, andthe catheter flexes in the flex points 150 between the modules 120.

In FIGS. 7 and 8, an inner catheter tube 430 is depicted havingconductive ridges, e.g., conductive ridge 440. When the inner cathetertube 430 is assembled within the catheter 10, the conductive ridgesserve to make electrical connections between the manifold 30 (or theelectrical connector 50) and the cMUT devices. The conductive ridges areformed, for example, as bumps in the catheter tubing that are thencoated with conductive material, or they can be part of a compositeextruded tube. In addition, they can simply be conductive stripesdeposited onto the tube 430 by methods known in the art. In the tube 430depicted in FIGS. 7 and 8, twelve of the conductive ridges deliver an ACsignal to a corresponding one of each of twelve circumferentiallyarrayed membranes within a module. It will be appreciated that anysuitable number of signal conductive ridges are used to couple signalsources/sinks to corresponding ones of the cMUT membranes.

A majority of the conductive ridges are used to carry signalscorresponding to ultrasound echoes that are created by the cMUT devices.In addition, there is an electrical ground conductive ridge 460 and a DCbias conductive ridge 470. The potential between the ground conductiveridge 460 and the DC bias conductive ridge 470 serves to maintain thecMUT membranes in a desired operational state. Thepositions/functionality of the various signal/conductive linescorresponding to the ridges differs in accordance with variousembodiments. It can be seen in FIG. 8, that the inner tube 430 can havea ridged section 480 and a non-ridged section 490. The non-ridgedsection 490 has a lower profile, and can be used as the catheter distaltip 130, as depicted in FIG. 2. This also facilitates easier tip bondingduring manufacture of the catheter. This is an efficient design becauseno conductive lines are needed at the very tip of the catheter, distalto the cMUT section, unless there any additional devices needed at thevery tip.

FIG. 9 shows an exemplary embodiment of conductive pathways to and froma cMUT membrane array of a cMUT ring 280 according to a firstembodiment. Membranes, such as membrane 290 are covered with a circulardeposition of conductive material. The conductive material, by way ofexample, is continuous with a conductive path (e.g., path 520 formembrane 290). Preferably, the conductive patterns on a face 530 aredeposited in one operation. Continuous with the conductive path 520 is aconductive stripe 500 on the inner wall of the cMUT ring 280. When thering 280 is attached to the inner tube 430 (see, FIGS. 7 and 8), one ofthe conductive ridges (e.g., ridge 440) contact, or are conductivelybonded to, the conductive stripe 500. This allows a signal sent alongthe path of a conductive ridge to be electrically coupled to theconductive material on the membrane 290. As depicted in FIG. 9,conductive stripe 510 is coupled to either the ground contact of thecMUT ring or the DC bias contact of the cMUT ring.

FIG. 10 is a semi-transparent view presenting two sides of the cMUT ring280 of the exemplary embodiment presented in FIG. 9. As depicted, afront facing side 590 has a first conductive path 550 associated with afirst membrane 530, and a second conductive path 570 is associated witha second membrane 640. A back facing side 600 has a first membrane 540associated with a first conductive path 560, and a second conductivepath 580 is associated with a second membrane 630. As depicted in FIG.10, membrane 530 and membrane 540 do not share the same conductivestripe because their respective conductive paths are angled in differentdirections. For example, the conductive path 550 of membrane 530 sharesa conductive stripe 620 with conductive path 580 of a different membrane630. The conductive path 560 of membrane 540 shares a conductive stripe610 with the conductive path 570 of a different membrane 640. In thismanner, it may be possible to reduce acoustic cross-talk associated withdrums on direct opposite sides of the cMUT ring. In other embodiments,the conductive paths associated with membranes on direct opposite sidesof the cMUT ring share the same conductive stripe. In yet anotherembodiment, the drums on one side of the cMUT ring are located atdifferent circumferential angular locations on the ring than the drumson the other side of the ring, for example, skewed 15° out of phasealong the opposing surfaces of the ring 280. This may also help toreduce acoustic cross-talk.

FIG. 9A shows a cMUT ring similar to that of FIG. 9, but includes acircumferential conductive stripe 635 that is connected to all of themembranes 290 by individual conductive bridges 645. This illustrates anembodiment suitable for connecting all of the membranes of a face to asingle terminal. The signal associated with the single terminal is, byway of example, an electrical ground, a DC bias Voltage or an excitationVoltage, depending upon the configuration. Alternatively, the singleterminal is able, for example, to signally communicate with multipleones of the transducer membranes via the conductive stripes, such asconductive stripe 500 (see, FIG. 9) on the inner wall of the transducermodule rings.

FIG. 11 shows a semi-transparent side view of the exemplary cMUT ring280, of the embodiment depicted in FIGS. 9 and 10, showing thearrangement of the drums. The cMUT ring 280 is formed, for example, ofsilicon or other semi-conductor material. It will be appreciated thatthe cMUT ring 280 is constructed of any one of suitable alternativematerials. The cMUT ring 280 is two-sided and includes a total of 24drums, formed by 12 membranes and 12 corresponding cavities on eachside. The two sides of the cMUT ring are separated by a common wall 660which supplies sufficient acoustic isolation so that it minimizesacoustic coupling between any membranes disposed on opposing sides ofthe ring 280. The common wall 660 forms the bottom of each drum shell.The sides of the drum shells include a combination of thecircumferential outer wall 650, a circumferential inner wall (notvisible in this longitudinal section) and a series of internal walls,e.g., wall 670. A first drum is depicted on a first side by first cavity690 and first membrane 680 disposed over the first cavity 690. A seconddrum is depicted on a second side by a second cavity 700 and a secondmembrane 710.

Another embodiment of a reflective cMUT IVUS catheter is shown in FIG.12. Modular series 720 consists of single-sided cMUT rings 730 andsingle sided sonic mirrors 740. The one-sided configuration of FIG. 12is potentially easier and relatively less inexpensive to produce inlarge quantities than the embodiment depicted in FIG. 5, and allows forparticularly flexible catheter-mounted probes due to the smallerrequired thickness of the elements. For example, the elements (modules)of the illustrated alternative embodiment have a thickness of between0.005 and 0.050 inches, and in some embodiments more particularlybetween 0.007 and 0.020 inches. By design, the axial image planes areequidistant from each other. In the previously described two-sidedembodiment, the cMUT ring and mirror are each manufactured at specificwidths in order to assure completely equidistant image planes.

FIG. 13 illustrates a one-sided cMUT ring 770 having an outercircumferential wall 750, an inner circumferential wall (not visible inthis figure) and a series of internal walls 780. There is also a backwall 760, because there are no membranes on the back side of the ring770. Also shown are a cavity 790 and a corresponding membrane 800.

FIG. 14 depicts an alternative embodiment of a drum pattern on a cMUTring. The cMUT ring diameter is, by way of example, similar to the cMUTrings in the previously described embodiments of a cMUT ring (forexample, about 0.038 inches in some embodiments), but in this case, thedrum diameters may be smaller. By way of example and not limitation, thedrum diameters in this embodiment are approximately 0.002inches—compared to 0.006 inches in the previously described embodimentsincluding a single circle of transducer elements. In the illustratedembodiment of FIG. 14, the drum pattern consists of three circles of 30drums 820 for a total of 90 drums. In other embodiments, the outer rowcan be more densely/efficiently filled (than the illustrated embodimentin FIG. 14) so that it has a greater number of drums than the inner row.It will be appreciated that the drums may be of any suitable size andany suitable quantity of drums are disposed on the cMUT ring in anysuitable pattern in accordance with alternative embodiments.

FIG. 15 shows a reflective cMUT IVUS catheter that delivers and receivesultrasonic waves in angular paths so that it can be used for bothside-viewing and forward-viewing. Forward viewing can be especiallyuseful in applications such as chronic total occlusions and theplacement of a guidewire into structures of the heart, such as thecoronary sinus, a pulmonary vein, or defects such as a patent foramenovale (PFO) or an atrial septal defect (ASD). In the chronic totalocclusion application, the guidewire lumen can be used to deliver aguidewire through the occlusion while the catheter images in the forwarddirection to assure that the guidewire is not passing through the wallof the artery. In this type of catheter, it may be desirable to have theproximal guidewire port located outside of the patient in the manifoldso that multiple guidewires can be exchanged as theexamination/procedure progresses.

Another example of a forward looking application is in IntracardiacEchocardiography (ICE). For example, a catheter can be placed so thatthe tip is in the right atrium and, while forward-viewing, the coronarysinus can be identified (by either blood flow or tissue imaging) and aguidewire can be delivered into the coronary sinus. This allows for thesubsequent delivery of bi-ventricular pacing leads. In addition, thecatheter is, for example, combined with a steering mechanism, whichallows the tip to be aimed in the desired direction. A similar system isused for placing a guidewire into a pulmonary vein, a PFO, ASD or otherheart structures or defects. The distal end 830 of a side and forwardviewing reflective cMUT IVUS catheter comprises flat sonic mirrors,e.g., sonic mirror 840 and cMUT rings 850 having a beveled face. Thebeveled face is shown in FIG. 16. The membranes, such as membrane 880,are disposed at any suitable angle, including but not limited to anangle such as 45°. As shown in FIG. 15, emitted ultrasonic waves 860reflect off the sonic mirror surface in a reflective angle that has bothside and forward components. A reflected wave 870 is thereforepotentially able to penetrate tissue that is located both to the sideand forward relative to the catheter.

FIG. 17 shows a catheter that is firing more than one of its cMUT rings.The ultrasound field 890 potentially has a funnel shape directed forwardrelative to the catheter, while still imaging tissue to the side.

In FIG. 18, the exemplary catheter is constructed such that it has adistal cMUT ring 900 that can fire independently of the other cMUTrings. The forward-viewing ultrasound field 910 is suitable for aforward-viewing application, such as total occlusions. The distal cMUTring 900 is made differently from the other cMUT rings on the catheter,for example, with differing membrane constructions, so that it vibratesat a lower frequency, and thus, penetrates deeper into the tissue. Anexemplary frequency is chosen from within the range of 2 MHz to 20 MHz,for coronary CTO applications. It will be appreciated that the cathetermay fire any suitable number of rings at a time, including all of therings at or approximately at the same time, to image tissue disposednear the front and/or the side of the catheter.

It is noted that cMUT membranes may be any suitable size including asthin as only a few microns, although for purposes of explanation, theymay be shown larger than scale in the Figures.

FIG. 19 depicts an embodiment of a side-viewing/forward-viewing catheterthat only has one cMUT ring 920 and one mirror 930, and is preferablyused for forward-viewing. Because of the relatively short length of theimaging section of this catheter, the distal end is potentially veryflexible. The catheter embodiment depicted in FIG. 19 is ideal forforward-viewing during a chronic total occlusion procedure. A guidewireor a series of guidewires are used down the guidewire lumen of thecatheter while forward-viewing the occlusion. As the guidewirepenetrates further, the catheter is advanced along behind the guidewirewith relative ease because of its excellent flexibility and tapered tip.

The sonic mirrors of the cMUT devices described herein are, for example,constructed of a very dense material which serves as an efficientreflector of ultrasound. In addition, it is desirable in someembodiments that the sonic mirrors have a very smooth surface withsurface imperfections no more than one-tenth of an ultrasoundwavelength. An example of an ideal material is stainless steel, whichhas an acoustic impedance significantly higher than blood or saline, andcan be polished to a mirror-smooth surface. In addition, stainless steelis a common material used in intravascular catheters. The nitride layerof the cMUT drum is generally less than a wavelength thick, so it doesnot serve as a true acoustic medium itself. It will be appreciated thatthe sonic mirror may be constructed of any suitable material.

FIG. 20 shows another embodiment of a cMUT IVUS catheter withoutmirrors, which need not utilize a reflective surface on the catheter.Each cMUT ring 940 has a series of radially oriented drums withmembranes, e.g., membrane 950. The membranes are directly side-firing,and the dimensions of the elements and the catheter itself are, forexample, on the same order as previously discussed reflective cMUT IVUSembodiments.

Turning to FIG. 23, a cross-section of the cMUT ring of this direct cMUTIVUS catheter demonstrates the arrangement of the drums, e.g., drum 960along the outer portion of the cMUT ring 940. In this embodiment, theconductive stripes in the inner diameter of the cMUT ring 940 are, forexample, similar to the cMUT rings of the reflective cMUT IVUScatheters. However, the conductive paths between the conductive stripesand the membranes 950 are carefully configured so that they do notcontact the conductive paths of another cMUT ring. In furtherembodiments, the conductive paths are formed internally of the cMUTring. In FIG. 23, the surface of the membranes 950 is flat and does notexactly follow the rounded, continuous circumferential contour of theouter surface of the cMUT ring 940. Alternatively, the cMUT ring can beformed so that the membranes follow exactly the circumferential contourand maintain a consistent thickness throughout.

FIGS. 21 and 22 depict additional embodiments of a direct cMUT IVUScatheter similar to the embodiment described in FIG. 20. In place of thesingle drum in each of the dodecahedral sides, there is instead an arrayof seven smaller drums. The seven drums create a circular array. It willbe appreciated that any suitable number of smaller drums may form thearray. Furthermore, in other embodiments, the array is rectangular, aparallelogram, an arrow shape, or any other suitable shape. FIGS. 21 and22 differ by the orientation between adjacent cMUT rings. In FIG. 21,all of the rings 980 have the same circumferential orientation. Contourline 985 is aligned for all rings. In FIG. 22, the orientation of eachconsecutive ring is staggered, in this case, by 15°. First contour line987 for first ring 980 b is 15° from second contour line 982 of secondring 980 a. In this manner, a more complex ultrasonic field may becreated to add more detail to the imaging data. It will be appreciatedthat the rings may be staggered at any suitable angle relative to oneanother.

Referring again to FIG. 22, some embodiments comprise integratedcircuits to communicate with and/or control the transducer elements. Theintegrated circuits, in addition to reducing the number of wires alongthe length of the catheter, also potentially carry out signalamplification and filtering as well as controlling firing sequences ofthe sets of transducer elements on the device. As shown, the integratedcircuits 992 are, for example, disposed near the distal end of thecatheter shaft near the cMUT rings, e.g., ring 980. The integratedcircuits 992 selectively multiplex/route the signals from the transducerrings 980 to a substantially smaller number of wires running along thecatheter to its proximal connector interface.

In the illustrative embodiment, there are, for example, ten integratedcircuits 992, and each circuit 992 is disposed as a respective one ofthe ten angled surfaces on the catheter shaft. Each of the tenintegrated circuits 992 communicates with one of the ten cMUT rings 980.It will be appreciated that a suitable number of integrated circuits aredisposed at suitable locations on the catheter to communicate with anysuitable number of cMUT rings and/or drums. For example, in otherembodiments, integrated circuits are mounted upon each ring, e.g., ring980 to communicate with the corresponding membrane(s) on the respectivering.

Another embodiment of a cMUT ring 945 for a direct cMUT IVUS catheter ispresented in FIG. 24. In this embodiment, a honeycomb pattern of drums,e.g., drum 965, are arrayed around the circumference of the cMUT ring945. In this specific configuration there are alternating rows of elevenand ten drums. With a total of 144 rows, there are a total of 1512drums. The necessary multiplexers may be cylindrically configured withinthe substrate of the cMUT ring 945 using conventional semi-conductorfoundry fabrication techniques.

In addition to rings, the cMUT modules are configured in other suitableshapes that allow for a more non-symmetric catheter cross section. Theseinclude, but are not limited to, elliptical cross-sections, teardropcross-sections and rectangular cross-sections. The membranes of the cMUTrings are not limited to the circular shape or honeycomb patternpresented here, but can also have a variety of other smooth andpolygonal shapes. In addition, it will be appreciated that a suitablenumber of drums are arranged in a suitable pattern in accordance withvarious alternative embodiments.

Furthermore, with regard to the mirror/reflectors, in accordance withvarious embodiments, instead of the beveled angle on the mirrors or cMUTrings, the mirrors and cMUT rings may be other suitable shapesincluding, but not limited to, concave or convex shapes.

It will also be appreciated that the drum membrane is potentiallydisposed at any suitable angle relative to a longitudinal axis of thecatheter. By way of example and not limitation, the angle may be between5° and 85°, between 25° and 65°, or between 40° and 50°. Similarly, thesonic mirror may have a surface for reflecting ultrasonic waves that isdisposed at any of a wide range of suitable angles relative to thelongitudinal axis of the catheter.

Besides intravascular ultrasound, other types of ultrasound catheterscan be made using the teachings provided herein. By way of example andnot limitation, other suitable types of catheters includenon-intravascular intraluminal ultrasound catheters, intracardiac echocatheters, laparoscopic, and interstitial catheters.

Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein.

Any references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein (including any references contained therein).

Illustrative embodiments of a cMUT-based probe are described herein.Variations of the disclosed embodiments will be apparent to those ofordinary skill in the art in view of the foregoing illustrativeexamples. Those skilled in the relevant art will employ such variationsas appropriate, and such variations, embodied in alternativeembodiments, are contemplated within the scope of the disclosedinvention. The invention is therefore not intended to be limited to theexamples described herein. Accordingly, this invention includes allmodifications and equivalents of the subject matter recited in theclaims appended hereto as permitted by applicable law. Moreover, anycombination of the above-described elements in all possible variationsthereof is encompassed by the invention unless otherwise indicatedherein or otherwise clearly contradicted by context.

1-30. (canceled)
 31. An ultrasound catheter for insertion intovasculature, the catheter comprising: an elongate flexible shaft havinga proximal end and a distal end; a capacitive microfabricated ultrasonictransducer device mounted on the shaft near the distal end, wherein thetransducer comprises a drum structure having a membrane disposed suchthat a face of the membrane is oriented to emit ultrasound waves; and asonic reflector mounted on the shaft adjacent to and separate from thedrum structure such that the sonic reflector is movable relative to thetransducer about a flex point, the sonic reflector comprising areflective surface and positioned such that the reflective surfaceredirects the ultrasonic waves to and from the transducer.
 32. Theultrasound catheter of claim 31, wherein the membrane of the transducerand the reflective surface of the sonic reflector are oriented tofacilitate side viewing.
 33. The ultrasound catheter of claim 32,wherein the face of the membrane is oriented to emit the ultrasoundwaves substantially along a longitudinal axis of the flexible shaft. 34.The ultrasound catheter of claim 31, wherein the membrane of thetransducer and the reflective surface of the sonic reflector areoriented to facilitate forward viewing.
 35. The ultrasound catheter ofclaim 34 wherein the face of the membrane is oriented to emit theultrasound waves in a direction that is not parallel to a longitudinalaxis of the flexible shaft.
 36. The ultrasound catheter of claim 35,wherein the face of the membrane is oriented to emit the ultrasoundwaves at an angle greater than 5 degrees and less than 85 degreesrelative to the longitudinal axis of the flexible shaft.
 37. Theultrasound catheter of claim 31, wherein the transducer is formed withina ring.
 38. The ultrasound catheter of claim 37, wherein the transducercomprises a plurality of drum structures and wherein at least one of theplurality of drums is disposed on a first face of the ring andconfigured to emit ultrasound waves proximally in a directionsubstantially parallel to a longitudinal axis of the flexible shaft andat least one of the plurality of drum structures is disposed on a secondface of the ring and configured to emit ultrasound waves distally in adirection substantially parallel to the longitudinal axis of theflexible shaft.
 39. The ultrasound catheter of claim 31, wherein theflexible shaft comprises a guidewire lumen
 40. An ultrasound catheter,comprising: an elongate flexible shaft having a proximal end and adistal end; a plurality of capacitive microfabricated ultrasonictransducer modules mounted on the shaft near the distal end and spacedapart from one another such that the shaft is able to flex between pairsof the transducer modules, wherein each of the plurality of transducermodules comprises a plurality of drum structures configured to emitultrasound waves, and a plurality of sonic reflectors mounted on theshaft adjacent to the plurality of transducer modules such that theshaft is able to flex, each of the plurality of sonic reflectorscomprising at least one reflective surface, the at least one reflectivesurface configured to redirect ultrasonic waves to and from theplurality of drum structures of an adjacent transducer module.
 41. Theultrasound catheter of claim 40, wherein the plurality of drumstructures of the transducer modules and the reflective surfaces of theplurality of sonic reflectors are oriented to facilitate forwardviewing.
 42. The ultrasound catheter of claim 40, wherein the pluralityof drum structures of the transducer modules and the reflective surfacesof the plurality of sonic reflectors are oriented to facilitate sideviewing.
 43. The ultrasound catheter of claim 40, wherein the pluralityof drum structures of the transducer modules and the reflective surfacesof the plurality of sonic reflectors are oriented to facilitate side andforward viewing.
 44. The ultrasound catheter of claim 40, wherein atleast one of the plurality of sonic reflectors includes a pair ofreflective surfaces, one of the pair of reflective surfaces configuredto redirect ultrasonic waves to and from the plurality of drumstructures of an adjacent transducer module on a first side of the sonicreflector and the other of the pair of reflective surfaces configured toredirect ultrasonic waves to and from the plurality of drum structuresof an adjacent transducer module on a second side of the sonicreflector, opposite the first side.
 45. The ultrasound catheter of claim40, wherein the plurality of transducer modules and the plurality ofsonic reflectors are mounted on the shaft in an alternating pattern. 46.The ultrasound catheter of claim 45, wherein the plurality of transducermodules are separate from the plurality of sonic reflectors such thatthe plurality of transducer modules are movable with respect to theplurality of sonic reflectors.
 47. The ultrasound catheter of claim 45,wherein each of the plurality of transducer modules is in a fixedrelationship with respect to at least one of the plurality of sonicreflectors.
 48. The ultrasound catheter of claim 47, wherein each of theplurality of transducer modules is a one-sided ultrasound transducer.49. The ultrasound catheter of claim 48, wherein the plurality oftransducer modules are configured to emit ultrasound signals in a distaldirection parallel to a longitudinal axis of the flexible shaft.
 50. Theultrasound catheter of claim 48, wherein the plurality of transducermodules are configured to emit ultrasound signals in a proximaldirection parallel to a longitudinal axis of the flexible shaft.